Preheat your oven to 170°C (340°F). Sift together cake flour, cocoa powder, baking soda, and baking powder into a bowl. Add 55g sugar and salt, whisking to combine.

In a separate bowl, mix egg yolks, vegetable oil, milk, and vanilla extract until smooth. Combine this with the dry ingredients, stirring until just blended.

Beat the egg whites with lemon juice until foamy. Gradually add 60g sugar, continuing to beat until stiff peaks form. Gently fold the meringue into the batter in thirds, being careful not to deflate the mixture.

Pour the batter into an ungreased chiffon cake pan. Bake for about 35-40 minutes, or until a skewer inserted into the center comes out clean. Invert the pan and cool completely.

Mix cornstarch with water until smooth. Combine this slurry with evaporated milk, sugar, cocoa powder, and agar-agar in a saucepan over medium heat. Stir constantly until the mixture thickens. Remove from heat and let cool slightly.

Beat the butter until creamy, then gradually incorporate the cooled chocolate mixture until the frosting is smooth and spreadable.

Once the cake is cool, use a serrated knife to level the top if needed. Apply a generous layer of chocolate frosting over the top and sides of the cake. Use a spatula or knife to create swirls or patterns.

Let the cake set in the refrigerator for at least an hour before slicing. Serve chilled for a refreshing contrast, or at room temperature for a softer texture.